The present invention relates to a receiver for an amplitude modulated signal and more particularly to tracking the amplitude of an incoming signal and recovering the desired signal (amplitude) from distortion imposed by the transmission channel by reconstructing the channel-induced amplitude distortion.
Communication and telecommunication devices use a variety of modulation techniques for transmitting information. Commonly employed techniques include frequency modulation (FM), phase shift keying (PSK) and quadrature amplitude modulation (QAM). As the terms imply, FM involves modulating the frequency of the carrier signal, PSK involves modulating the phase, and QAM, involves modulating both the phase and the amplitude of a carrier signal.
A pervasive problem in telecommunications is the distortion imparted onto the signal during transmission and the need to recover, as accurately as possible, the desired signal from the noise, attenuation and distortion imposed by the transmission channel. This problem is particularly severe with radio telecommunication devices such as two-way radios and cellular phones.
Channel-induced distortions imposed upon the amplitude of the transmitted signal can be particularly difficult to compensate for. In a QAM system, the carrier signal will be modulated to two or more discrete amplitudes. For practical circuit design, these discrete amplitude levels cannot be greatly far from one another and hence the distortion imposed by the transmission channel can cause errors on the receiving side when trying to determine at what amplitude level the signal was actually sent.
With differential QAM systems, the information in the signal is encoded in the difference in the amplitude of adjacent sample points (symbols) of the signal, rather than in the absolute amplitude of the symbols themselves. Differences in the amplitude of the received signal may be interpreted on the receiving side as differences in the amplitude of the transmitted signal, such as a jump from one ring of the QAM constellation to the next, when in fact, the amplitude change was due to fading of the signal in the transmission channel. On the other hand, an intended jump from one ring of the constellation to another, might be lost due to channel fading causing the overall received signal amplitude to remain unchanged (or in fact going in the opposite direction of the transmitted amplitude change).
One prior art approach to recovering signal amplitude is the use of pilot symbols in the transmitted signal. These pilot symbols are of a known configuration (i.e. known location in a frame) and known amplitude (and phase). The receiver recognizes the pilot symbols and knows at what amplitude level the pilot symbol was transmitted. Knowing the level at which the pilot symbol was transmitted and knowing the amplitude of the actual received symbol, it is a relatively easy calculation to determine the distortion caused by the channel and to impose a compensation signal onto the received signal to remove the channel effects from it. Multiple pilot symbols need to be inserted into the transmitted signal periodically, as the channel effects are time variant.
With mobile telecommunication devices, channel effects can change rapidly (for example with a cellular phone in a car traveling at highway speeds). For this reason, pilot symbols must be inserted into the transmitted signal often. A shortcoming with the use of pilot symbols is that the pilot symbols add to the xe2x80x9coverheadxe2x80x9d of the transmitted signal (i.e. the portion of the signal associated with synch and control, not associated with the actual information such as data or voice information being transmitted). Pilot symbols, therefore, lower the useful bandwidth of the telecommunication system.
A need exists in the prior art for a system in which the magnitude of an incoming amplitude-variant signal can be tracked, and channel distortions to the channel removed, without significantly decreasing the information carrying capacity, or bandwidth of the transmitted signal.
The present invention provides for processing a received signal in order to more accurately decode the received signal for information content. The received signal has an information content component and a channel-induced distortion component. The information content component is comprised, at least in part, in variations in transmitted signal magnitude. The information content component of the received signal is removed from the received signal by canceling magnitude variations in the received signal arising from the information content. In this way, the remaining signal""s magnitude variations are those that result from channel-induced distortion. This reconstructed channel-induced distortion can then be used to generate a compensation factor that can be applied to the received signal to remove the effects of the channel-induced distortion from it, thus allowing a more accurate decoding of the received signal.